Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 390

BB-SFG, we have investigated CO adsorption on smooth polycrystalline and single-
crystal electrodes that could be considered model surfaces to those applied in fuel cell
research and development. Representative data are shown in Fig. 12.16: the Pt nano-
particles were about 7 nm of Pt black, and were immobilized on a smooth Au disk. The
electrolyte was CO-saturated 0.1 M H 2 SO 4 , and the potential was scanned from 20.19
V up to 0.64 V at 1 mV/s. The BB-SFG spectra (Fig. 12.16a) at about 2085 cm 21 at
20.19 V correspond to atop CO [Arenz et al., 2005], with a Stark tuning slope of
about 24 cm 21 /V (Fig. 12.16b). Note that the Stark slope is lower than that obtained
with Pt(111) (Fig. 12.9), for reasons to be further investigated. The shoulder near 2120
cm 21 is associated with CO adsorbed on the Au sites [Blizanac et al., 2004], and the
broad background (seen clearly at 0.64 V) is from nonresonant SFG. The data shown
in Figs. 12.4, 12.11a, and 12.16 represent a link between smooth and nanostructure
catalyst surfaces, and will be of use in our further studies of fuel cell catalysts in the
BB-SFG IR perspective.
12.5 SFG OF BIMETALLIC ELECTRODES: ADSORPTION AND
OXIDATION OF CO ON Pt(111) / Ru
12.5.1 Correlation between Infrared Spectra and Reactivity
By employing spontaneous deposition to produce Ru-covered, Pt single-crystal elec-
trodes [Crown et al., 2002], we obtained Pt(111)/Ru surfaces with a Ru coverage of
approximately 0.2 monolayers (ML) (Fig. 12.17). Next, using CO-saturated, 0.1 M
H 2 SO 4 solutions and Pt(111)/Ru surfaces such as that shown in Fig. 12.17, we pro-
duced CO chemisorption layers in which CO was chemisorbed on both Ru and Pt
sites at full coverage [Lu et al., 2002]. After flushing the cell with clean 0.1 M
H 2 SO 4 electrolyte, we conducted voltammetric oxidation of the CO under voltam-
metric conditions, into the pure H 2 SO 4 electrolyte (Fig. 12.18). The current versus
electrode potential measurements yielded two well-resolved current - potential peaks
at 0.14 and 0.24 V vs. Ag/AgCl, at 1 mV/s [Lu et al., 2002; Tong et al., 2002].
Among other things (see below), the voltammogram in Fig. 12.18 demonstrates the
high quality of the electrode surface, and confirms the cleanliness of our electrochemi-
cal systems.
Others [Massong et al., 2000] as well as us [Tong et al., 2002] have reported on the
split in the voltammogram of the type shown in Fig. 12.18, and it is now fully acknowl-
edged that the more negative cyclic voltammogram peak originates from the oxidation
of CO chemisorbed on Ru sites, while the more positive peak comes from the
oxidation on pure (not Ru-covered) Pt sites (see [Lu et al., 2002; Tong et al., 2002]
and references therein). We may now hypothesize that the vibrational spectra
[Lu et al., 2004; Friedrich et al., 1996] obtained from such CO-covered Pt(111)/Ru
surfaces should follow the voltammetric distribution of the CO stripping peaks of
Fig. 12.18. That is, on the positive-going voltammetric scan, the spectral
amplitudes for CO adsorbed on Ru sites should disappear before the amplitudes
from CO on the Pt sites can be reduced (before the ultimate disappearance at higher
anodic potentials).